The present invention relates to a process and apparatus for purifying carbon dioxide using a sequence of adsorbents including a metal oxide, silica gel, and activated carbon.
Carbon dioxide has been known for centuries, with Pliny the Elder describing it in the context of poisonous vapors coming from caverns. In the seventeenth century, Van Helmont obtained carbon dioxide by such means as fermentation and acidification of carbonates, and also studied many of its properties. Later it was recognized as an acidic gas but it was not until the end of the eighteenth century that Lavoisier recognized it as a compound of carbon and oxygen of a given proportion.
Since mineral waters (solutions of carbon dioxide in water) were thought to have medicinal properties, there was from the onset an incentive to commercially exploit carbon dioxide. Farady made liquid and solid carbon dioxide using a hydraulic pump and studied solid carbon dioxide as a refrigerant. Its uses over time have proliferated to include such diverse applications as beverage carbonation, chemical manufacture, fire fighting, food freezing, greenhouses, oil well secondary recovery, as an atmosphere in welding, and even more recently in supercritical extraction processes.
The bulk of carbon dioxide is generated from ammonia and hydrogen plants as process gas carbon dioxide resulting from the reaction between hydrocarbons and steam. The carbon dioxide produced by such methods has a high purity but may contain, inter alia, traces of hydrogen sulfide, sulfur dioxide, and hydrocarbons including high molecular weight hydrocarbons from compression and pump oils which are particularly detrimental to its use in the semiconductor industry. Carbon dioxide use in the semiconductor and silicon wafer production industries has seen much recent growth. However, these industries require extremely pure carbon dioxide in order to ensure that the products do not become contaminated by impurities that might be present in the carbon dioxide. It is particularly important that compounds containing about ten or more carbon atoms be removed from the carbon dioxide, and most important the compounds containing greater than about 15 carbon atoms be removed. This application is primarily concerned with purification of carbon dioxide for use in the semiconductor and silicon wafer production industries, where high purity carbon dioxide is required.
The most commonly used purification method is treatment with potassium permanganate or potassium dichromate. Both potassium permanganate and potassium dichromate are active oxidizing agents, consequently scrubbing generally results in oxidation of unwanted materials. In the case of hydrogen sulfide as a contaminant, oxidation results in the formation of sulfur that is readily removed as a solid. Where it is necessary to also remove water from the carbon dioxide, a separate drying step over alumina has sometimes also been used. Nonetheless, the presence of residual impurities often remains a problem in providing even food-grade carbon dioxide which meets the Compressed Gas Association commodity specifications much less the particular needs of the semiconductor and silicon wafer production industries.
Although the commercial production of carbon dioxide has been ongoing for many years now, and although the purification of the carbon dioxide has been the subject of many efforts, nonetheless a truly high purity carbon dioxide is expensive to produce and not widely available. In answer to this need, the present invention uses a combination of adsorbents to very effectively and very efficiently remove impurities such as water, sulfur-containing compounds, nitrogen-containing compounds, and hydrocarbons. In particular, the present invention provides a combination of adsorbents that are uniquely suited to removing compounds containing greater than about ten carbon atoms.
The invention described within is a method of purifying liquid or gaseous carbon dioxide. In particular, the present invention is a process for preparing high purity carbon dioxide suitable for use in the semiconductor and silicon wafer production industries. An embodiment comprises passing gaseous carbon dioxide through one or more beds containing (1) a metal oxide, (2) silica gel, and (3) activated carbon. The silica gel may be commonly available silica get such as Grade 3, 8 mesh silica gel, or may be alumina- or magnesia- stabilized silica gel, mixtures thereof, or a bed of each type of silica gel may be used. In a more specific embodiment a molecular sieve such as 4A molecular sieve may be used as the first bed to remove water. In another specific embodiment, silicalite may be used as a bed to remove a wide variety of low molecular weight organic compounds and oxygen-containing compounds. More specific embodiments of the invention involve additional adsorbents such as ZSM-5, ZSM-12, ZSM-23, silver-exchanged faujasite, reduced metal on matrix material, and combinations thereof. Other embodiments and applications will be apparent from the ensuing description.